At the time of posting the question, no, there was no way to earn diamonds in LPFA, you always had to pay for them.

Now there's the wheel of fortune. Keep spinning it for several days until the option 'winning diamonds' gets enabled. This gives you a certain chance to win some of those gems each day from now on. However, if you fail to spin the wheel for even a single day you will have to restart with the wheel at day one to work your way up until diamonds become available for you again.

I know it's kinda tedious but the only way if you can't spend real money on diamonds.